What’s all the fuss about solar? Dissecting the photovoltaic integration with metal roofing—Part 2
Rob Haddock,
Posted
10/01/2008
In the last issue,
we covered the basics of photovoltaic power generation systems,
including collector types, system components, how the systems work
and some industry terminology (Design Ideas section, September
issue, page 34). In this issue we will focus on why standing seam
is indisputably the best mounting platform possible for PV. What
are the marked advantages of metal (that translate to cash) over
other roof types that are actually driving roof designs to metal
when power generation is a design objective?
Why Standing Seam On built-up and single-ply membrane roof types,
PV arrays are most often "rack"-mounted. An elaborate framework is
constructed of non-corrosive metals and then mechanically attached
through the roof membrane into the building structure. This method
has the obvious disadvantage of punching a lot of holes through the
roof for the PV attachment. Flashing holes in any roof type is
expensive and problematic. Often, roof warranties are nullified and
building owners must endure the attendant "finger pointing" when
the leaks begin. The usable life of PV is in the 25- to 35-year
range (depending on PV type and tolerable drop in efficiency).
Usable life of most roof membranes (other than standing seam) is 12
to 18 years. This means that the roof will require replacement
before the PV system-in some cases more than once. To do this, the
PV modules, together with their racking system, must all be removed
and then reinstalled following roof replacement, adding logistic
difficulty and considerable cost to an already expensive roof
replacement-not to mention the environmental impact of dumping
tear-off materials into the landfill.
The benefits of structural standing-seam metal roofing, or SSMR,
and PV are the methods of mounting the array and longevity of the
completed system. To begin with, the SSMR generally provides a
service life in excess of 40 years, outlasting the PV array. PV is
mounted on SSMR either by adhesion (thin film laminate) or
mechanically by seam clamps (thin film or crystalline modules).
Neither method is invasive to the SSMR roof. Depending on
wire-management techniques and other details, it is not only
possible but common practice to mount massive power generation
arrays on metal rooftops with zero penetration of the roof. By
mounting to a metal roof in this fashion, weather integrity and
roof warranties are left intact. Roof replacement prior to PV is
not an issue, and when the usable life of metal roofing and PV is
expired, all roof and PV materials are recyclable. If that isn't
enough, the serviceability and maintenance freedom of metal are
also far superior to other roof types.
On top of all the
foregoing benefits, the installation costs are also significantly
lower. Because the SSMR provides a grid work of beam like
configurations, the PV array conveniently mounts either by
lamination or mechanical attachment above the seams of the roof.
These methods are much less labor intensive and translate into
significant cost savings over other roof types. While conventional
racking system costs are typically in the range of 30 cents to 70
cents per watt, direct mounting costs on metal roofing are about 10
cents per watt-a savings of 20 cents to 60 cents. Expressing these
unit figures with respect to square-foot costs, one should remember
that most generation systems are in excess of 10 watts per square
foot. This puts cost savings when mounting to SSMR at $2 to $8 per
square foot over traditional flat roofing. Construction time is
also reduced, enabling earlier project completion.
All these advantages are so compelling that in many cases they can
drive roof selection to SSMR in the design stage of a project when
PV is an objective. If that's not convincing enough- there is still
more!
Other Beneficial Effects
PV modules (crystalline or thin film) that are frame-mounted above
the surface of the roof cast the roof's surface in shade while also
creating an airspace, or "plenum," that benefits from "stack
effect," pulling air through the space. These factors will keep
roof temperatures cooler in daylight hours and warmer in nighttime
hours, significantly reducing temperature differentials from day to
night. Such shading and plenum may well create the "ultimate" cool
roof, lowering temperatures far more than infrared reflective
pigments ever could. This effect also improves performance of the
PV from a power-output efficiency standpoint. (PV is more efficient
at cooler temperatures). When fitted with mechanically attached PV
arrays, high-end roof surface temperatures can be expected at more
than 55 F (13 C) cooler than normal for aged bare Galvalume steel
and painted surfaces not utilizing cool pigments. There is a
somewhat lesser disparity when cool pigments are utilized. The
results are summarized as follows:
1. The cooling load is reduced during summertime daylight hours due
to shading and plenum effects.
2. Wintertime heating load is reduced in nighttime hours due to
reduction of radiant heat loss from roof to sky because of the
radiant canopy effect. This, in turn, reduces convective heat loss
from building to roof.
3. The dimensional change of the panels is reduced due to both the
shading and nighttime canopy effects. This reduces severity of
thermal cycling of the roof, also reducing the wear and fatigue of
roof attachment components.
4. The PV operates more efficiently due to the cooling effect of
the plenum.
The above effects are applicable to PV modules that are frame
mounted above the surface of the roof, whether crystalline or thin
film. Films that are laminated directly to the roof panel have the
inverse effect.
Details of
Attachment
PV modules are typically attached above and planar to the SSMR
surface using aluminum seam clamps with round-point setscrews that
do not breach the metallic coatings of the panel seams. These
clamps engage the seam by pinching it within the clamp body.
Interface "hold-down" hardware is in turn bolted to the clamp body
and anchors the PV modules to the top of the seam clamps. Holding
strengths of these type of clamps are in many cases published and
can exceed the beam-strength of the panel seam, as well as its
attachment to the structure. It is common that solar modules are
installed in "landscape" orientation (long dimension traversing
seams) and "ganged" together with adjacent modules upslope and
downslope, sharing a mounting point and attachment clamp. Given a
module width of 3 feet (0.9 m) (+) and assuming industry standard
structural purlin spacing of 5 feet (1.5 m), the resulting
attachment frequency of PV modules to SSMR is equal to or greater
than the attachment frequency of SSMR to building structure.
SSMR manufacturers have used seam clamps to enhance wind-uplift
performance of SSMR roof panel systems by locking male and female
seam edges together with compression from the clamp. Some have also
used "deflection limiter" devices to restrain the upward bow of the
roof panel under winduplift loads. The seam clamp replicates this
performance when used for PV attachment. The nature of the
aluminum-framed PV module is such that it replicates the deflection
limiter. Given these facts, it may just be that the installation of
PV on the SSMR actually improves the behavior of the SSMR under
negative pressure experienced in a windstorm. This theory, however
logical, is also somewhat speculative as it has not been validated
by testing, nor is there an industry standard test protocol to do
so.
The Best Building Design for PV
There are some simple things that can make a building design more
"friendly" for PV installation. Of course the first step is to be
sure to use SSMR. Another obvious point is that the most beneficial
orientation for a PV array is south facing. Ridges that are
oriented east-west are strongly preferred over those oriented north
south, especially when steeper slopes are incorporated. A question
that always arises is, "What size system can be installed?" This is
governed by south-facing roof sizes more than any other single
factor; so single-slope design renders double the usable space of
gabled design- again when steeper slopes are employed. On very low
slope roofs, both roof planes might be used. It is helpful to
consult with a PV integrator in early design stages to ascertain
the best design while considering all factors-including the costs
of making the project "friendly" for the PV array. He can render
the ideal roof design and orientation-but then someone must also
evaluate economic consequences. It could be that the efficiency
gained by raising roof slope does not economically justify the cost
of raising the roof slope, given added construction costs,
additional space heating requirements of the higher ceiling and so
on.
With respect to roof slope, the optimal angle in the summer is
different than the optimal angle in the winter simply due to the
orientation of the sun, so calculations are done to come up with
the "weighted average," so to speak, or optimal angle. The ideal
slope also varies with latitude of the project site. In general,
slopes steeper than normal for the pre-engineered metal building
industry are better for PV arrays (somewhere around 30 degrees for
most of the U.S.), but this by no means precludes use of PV on
low-slope roofing. In fact, much is installed on low-slope roofing
just because of the availability of that roof inventory for
retrofitted PV systems. For new construction, the premium costs of
achieving optimal solar slope should be evaluated on a job-specific
basis and economically justified. Adding slope also helps to keep
the PV panels washed of dust and dirt and, therefore, performing
more efficiently. Rooftop equipment, appurtenances, parapets and
the like will shade certain areas of the roof, reducing usable
space for PV. Given the high cost of PV modules, they are generally
not used where they will be even partially shaded.
How Big? What
Kind? How Much?
Because the PV industry language is "watts" and construction
language is "square feet," it is helpful to relate the two. It is
common that covering the entire roof area available still does not
provide all the power desired, so getting the most output from the
space available is usually a primary consideration. Also, because
of the high cost of PV collector modules, "shading" of the modules
by parapet walls, adjacent structures, rooftop mechanical equipment
and so on must be carefully anticipated and always avoided. The
module needs every opportunity to produce power, and investing in a
system that will b even partially shaded at times is not a prudent
investment. This can mean all rooftop space is not suitable for PV
mounting.
To understand the proportionate values and size of PV arrays: A
30,000-square-foot (2,787- m2) unobstructed roof surface will
accommodate a system size of about 300 kilowatts (300,000 watts)
for crystalline and about 130 kilowatts (130,000 watts) for thin
film. At $6.50 per watt, the crystalline system will cost just
under $2 million-or $65 per square foot-often equaling or even
exceeding all other construction costs combined. The real service,
actual power outputs of the two systems (crystalline/ thin film),
may be a bit closer together than these figures look as the thin
film will outperform the crystalline in "low light" situations like
cloudy days and early morning light, rendering a higher number of
collection hours, albeit at lower collection rates. Economies of
scale also come in to play on PV systems-in a big way. In-place
costs of PV arrays vary from about $6/DC watt-for very large
systems-to double that for very small systems. Economic paybacks
are very contingent on government subsidies (both cash and tax
credit) and also local public utility electric rates. Heavily
subsidized large systems in areas that have high electric rates
have been said to pay back in as little as five years or less. Low
or no subsidy on a small system in an area with low electric rates
may render paybacks in the 30- or 40-year range.
Which is Best?
Crystalline or thin film? Amorphous silicon or cadmium telluride?
Laminate or framed module? It can be difficult indeed to sort out
the facts from sales rhetoric in any industry or market, and the
field of PV is little different. System selection should always be
based on real-world AC-power output, not just cost-per-rated-watt
output or standard-test-conditions watts. Accurate output forecast
data that considers appropriate projected system efficiency,
component efficiency, aging performance and other losses should
always be provided and considered. Data should always be system-
and site-specific, not generalized marketing statements. Some
companies even guarantee production rates. On steeply sloped roof
surfaces, aesthetics may also be a consideration.
One thing is certain: Initial PV costs for SSMR are significantly
lower than for other roof types. Total life-cycle cost analysis
that considers necessary roof replacement costs during the usable
life of the PV will also demonstrate that using metal roofing under
the PV not only conserves the environment but conserves the cash,
as well.
Rob Haddock is president of Metal Roof Advisory Group, Colorado
Springs, Colo. He is a well recognized authority on metal roofing,
a technical writer, trade curriculum author, inventor and educator.
Details are available at www.s-5.com.
www.s-5.com